The invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with input terminals for connection to a supply source, output terminals for connection to the lamp a switching device for operating the lamp at a nominal lamp voltage Vla during stable lamp operation and for generating an open voltage Vo at the output terminals before the lamp has ignited, an ignitor for generating an ignition voltage pulse when the voltage at the output terminals reaches a threshold value Vi, and a control circuit for controlling the switching device.
A conventional circuit arrangement, such as disclosed in European Patent No. 0401931 (corresponding to U.S. Pat. No. 5,068,572) is suitable for igniting and operating, inter alia, high-pressure sodium lamps and metal halide lamps. These lamps typically have a discharge vessel in which an electric discharge is maintained during operation and which is enclosed with intervening space by an outer bulb. An ignitor produces a high ignition voltage pulse (a few kV and more). The ignitor becomes operative through the choice of an open circuit voltage Vo across the output of a commutator being greater than the voltage threshold Vi for triggering ignition of the lamp, that is, Vo&gt;Vi. Once the ignitor becomes operative, an ignition voltage pulse is generated. The lamp will ignite and the voltage across the lamp will drop abruptly to around ten volts. As soon as a stable discharge is established in the lamp, the voltage across the lamp gradually rises to the lamp voltage Vla. The circuit arrangement is so designed that the lamp voltage Vla is lower than the voltage Vi, which in its turn is lower than the open voltage Vo.
A control circuit ensures that switching device of the circuit arrangement acts as a controlled current generator when in the stable operational state of the lamp. No more ignition voltage pulses are generated by the ignitor as soon as the voltage at the circuit arrangement connection terminals to the lamp drops to the lamp voltage Vla or lower. The generation of ignition voltage pulses after the lamp ignites is undesirable because this forms a load on the circuit arrangement. It is often suggested in the literature, therefore, to limit the operation of the ignitor by switching it off after a certain period of time has elapsed.
Conventional ignitors which are turned off after a prefixed period has elapsed have a number of disadvantages. These disadvantages include a comparatively high open voltage Vo remaining at the output terminals if the lamp has not yet ignited. The ignitor will also generate ignition voltage pulses after the lamp is extinguished, for example upon reaching the end of its operational life. High-pressure sodium lamps and metal halide lamps can ignite upon the application of ignition voltage pulses at the end of life after extinguishing and cooling-down. This gives rise to the characteristic flickering behavior of the lamp. Flickering is unpleasant to observe, but more importantly, is detrimental to the circuit arrangement and typically leads a considerable amount of interference radiation (radio interference, etc.).